The present invention generally relates to an improved digital receiver. More particularly, the present invention relates to a digital receiver with improved decision circuitry.
Quadrature Amplitude Modulation (QAM) represents one of the most popular modulation formats used in present-day digital communication systems. QAM is a signal format obtained by separately modulating the amplitude and phase of two components of a sinusoidal carrier differing in phase by 90 degrees. The two components of a QAM are referred to as an in-phase component (usually denoted I or P) and a quadrature component (usually denoted Q).
Generally, QAM covers the classes of Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK) and higher order QAM modulations. QPSK is widely used, for example, in the IS-95 cellular communication standard as well as in DirectTV. Additionally, modulation formats such as Minimum Shift Keying (MSK) are popular. MSK is also widely used, for example in the GSM cellular communication standard.
One of the key performance metrics of any digital receiver is the minimum Signal-to-Noise Ratio (SNR) of the received waveform it can operate at. Waveforms with a low SNR (signal power approximately the same as noise power) are prone to a high Bit-Error-Rate (BER). Generally, in low-SNR waveforms additional error, correction may be needed.
The operation of a typical digital receiver is adversely affected by noise. Thus, much effort in the field has been directed toward minimizing noise effects on communications signals and providing an increased SNR. Although increasing the power of the transmitted signal can raise the SNR in an environment with a static noise level, a practical upper limit on the power of the transmitted signal is often present. The upper limit on signal power may be influenced by such factors as, for example, the power capacity of the components of the transmitter. Also, the Federal Communications Commission (FCC) limits the maximum power of transmitters as part of their responsibility to allocate RF resources and to protect consumer health. Furthermore, the additional size and weight requirements of higher power components or power storage may provide the upper bound for transmitted signal power. Also, in a spread spectrum communications systems such as CDMA, a high SNR may not be feasible.
Thus, an improvement in SNR at the receiver generally yields a reduction in BER. Although, as mentioned above, the SNR is dependent upon both the transmitted signal power and the noise/interference environment of the transmission, the SNR may also be improved at the receiver. For example, high quality Band-Pass Filters (BPF) may be employed at the receiver to minimize the noise component of the SNR from outside of the frequency band of communication. The resultant, filtered signal may have a higher SNR and thus yield a lower BER. When thermal noise is the dominant noise, a BPF matched to the transmitted waveform provides the theoretical, maximum SNR.
However, high quality BPFs are expensive and complex and additional techniques for improving SNR (and thus the reliability of transmitted data) are constantly being sought. Furthermore, the additional techniques are, of course, preferably compatible with existing signal receiver front ends. An improved SNR may pave the way to less weighty, less power-consumptive, less expensive, and higher bandwidth communications systems. The resulting systems provide higher reliability and may further increase economic gains. In the past, however, higher SNR has typically required unacceptable increases in system cost, complexity, or unreliability.
Thus, a need has long existed for an improved digital receiver that yields an improved SNR at the receiver.
One object of the present invention is provide a digital receiver yielding an improved Signal-to-Noise Ratio (SNR) and a corresponding improved Bit Error Rate (BER).
Another objective of the present invention is to increase the reliability and to minimize the cost, size, weight, and complexity of hardware used to provide a digital receiver with an improved SNR.
It is also an object of the present invention to provide a digital receiver compatible with existing signal receiver front ends.
One or more of the foregoing objects are met in whole or in part by the improved digital receiver of the present invention. The present invention provides a digital receiver which first mixes an input analog waveform with a Intermediate Frequency (IF) local oscillator. The IF mixed signal is then band pass filtered and converted to a digital signal. The resultant digital signal is demultiplexed into several streams of digital pulses. The stream with the greatest maximum average power is selected by a detector. The selected stream is then provided to a maximum likelihood decision mapping circuit which selects the transmitted symbol and thus the transmitted bits.
The present invention may use the peak IF values as detector inputs. By sampling at an Intermediate Frequency (IF) instead of converting the received signals to baseband and extracting maximal per period values as inputs into the maximum likelihood detector, the digital receiver yields an improved SNR.
These and other features of the present invention are discussed or apparent in the following detailed description of the preferred embodiments of the invention.